Friday, December 18, 2009

Charbonneau et al. have detected a planet just six and a half times as massive as Earth - at a distance so close its atmosphere could be studied, and with a density so low it's almost certain to have abundant water.

The alien world known as GJ 1214b orbits a red dwarf star one-fifth the size of our own sun, 40 light-years away in the constellation Ophiuchus, the astronomers reported in Thursday's issue of the journal Nature.

A decade ago, the detection of the first transiting extrasolar planet provided a direct constraint on its composition and opened the door to spectroscopic investigations of extrasolar planetary atmospheres. As such characterization studies are feasible only for transiting systems that are both nearby and for which the planet-to-star radius ratio is relatively large, nearby small stars have been surveyed intensively. Doppler studies and microlensing have uncovered a population of planets with minimum masses of 1.9-10 times the Earth's mass (M_Earth), called super-Earths. The first constraint on the bulk composition of this novel class of planets was afforded by CoRoT-7b, but the distance and size of its star preclude atmospheric studies in the foreseeable future. Here we report observations of the transiting planet GJ 1214b, which has a mass of 6.55 M_Earth and a radius 2.68 times Earth's radius (R_Earth), indicating that it is intermediate in stature between Earth and the ice giants of the Solar System. We find that the planetary mass and radius are consistent with a composition of primarily water enshrouded by a hydrogen-helium envelope that is only 0.05% of the mass of the planet. The atmosphere is probably escaping hydrodynamically, indicating that it has undergone significant evolution during its history. As the star is small and only 13 parsecs away, the planetary atmosphere is amenable to study with current observatories.

Saturday, November 28, 2009

Last weekend and this week, the LHC has accomplished many tasks: it first circulated two beams in opposite directions, then made them collide in the heart of the four giant detectors and finally slightly increased their energy. Virtual champagne to the hundreds of people working nights and days to repair the machine, prepare it for new start-up and finally operate it.

Wednesday, October 28, 2009

(The first ion beam entering point 2 of the LHC, just before the ALICE detector October 23rd 2009)

This past weekend (October 23-25th) particles once again entered the LHC after the one-year break that followed the incident of September 2008.

Friday afternoon a first beam of ions entered the LHC's clockwise beam pipe through the TI2 transfer line. The beam was successfully guided through the ALICE detector until point 3 where it was dumped.

During the late evening on Friday, the first beam of protons also entered the LHC's clockwise ring and travelled until point 3. Saturday afternoon (October 24th), protons travelled from the SPS through the TI8 transfer line and the LHCb experiment, until point 7 where they were dumped.

All settings and parameters showed a perfect functioning of the machine, which is preparing for its first circulating beam in the coming weeks.-CERN News

Sunday, October 18, 2009

A strange halo cloud over Moscow had many in the Russian capital expecting a close encounter last Wednesday.

Millions witnessed an ominous ring-shaped cloud appear over Moscow’s western districts, prompting citizens to stop in their tracks to record the phenomenon and put it on YouTube.

The bizarre sight has taken the Internet by storm, so to speak.

However, scientists from the city’s weather forecast service dispelled fears of extra terrestrials landing in Red Square, saying the event was strictly environmental.

“It’s a purely optical effect, even if a spectacular one. You can see really strange things if you watch the clouds regularly,” weather officials told Russia’s Vesti 24. “Several air fronts have passed Moscow recently, including an inflow of cold air from the Arctic, and they combined to produce such a phenomenon

Indeed halo clouds have appeared elsewhere in the world, a notable example being the 'Dorset Doughnut' seen over Dorset, UK and captured in various images by the Cloud Appreciation Society.

We propose an experiment which consists of drawing a card and using it to decide restrictions on the running of Large Hadron Collider (LHC for short) at CERN, such as luminosity, and beam energy. There may potentially occur total shut down. The purpose of such an experiment is to search for influence from the future, that is, backward causation. Since LHC will produce particles of a mathematically new type of fundamental scalars, i.e., the Higgs particles, there is potentially a chance to find unseen effects, such as on influence going from future to past, which we suggest in the present paper.

Saturday, September 26, 2009

In the recent arXiv:0906.5135, Saridakis and Ward propose a novel dark energy candidate, based on the dynamics of ghost D-branes in compactified Type II string theory. This is an approach which is based on open string dynamics that envisions a post-inflation universe living on branes that wrap cycles in the compact space, with the GUT or Electro-Weak phase transition manifested geometrically.

Ghost D-branes are defined as Dp-branes with a Z_2 symmetry that can flip the signs of the NS-NS and RR sectors (arXiv:0601024[hep-th]). By using ghost D-branes, the authors are able to recover an effective dark energy behaving either as quintessence or as a phantom field, without the need of orientifolds.

Friday, September 25, 2009

It has been known for some time that minimal SUSY SU(5) is ruled out because proton lifetimes are less than the measured limits (t_p >= 6.6 x 10^33 yrs). In arXiv:0909.4105 [hep-ph], Abel and Khoze propose that the MSSM is a magnetic Seiberg dual of an unknown electric theory. Such a proposal would explain why the supersymmetric Standard Model appears to unify but the proton does not decay. The authors put forward a number of possible dual GUTs that may provide a consistent UV completion of the minimal SU(5) model, for example with SU(11)xSp(1)^3 and SU(9)xSp(1)^3 symmetry.

Wednesday, September 16, 2009

In arXiv:0907.3048, Nicolai et al. investigate cosmological singularity resolution in the context of an E_10 coset model. E_10 is the Kac-Moody big brother of E_8, the exceptional Lie group that was all the rage not long ago. The authors conclude there is a 'de-emergence' of space-time near the singularity. They also discuss observables near the singularity where they mention that the conserved E_10 Noether charges do constitute an infinite set of observables and their expectation values remain well defined in the deep quantum regime where the E_10/K(E_10) coset model is expected to replace space-time quantum field theory.

Thursday, June 25, 2009

As mentioned at RF, Gunaydin and Pavlyk have posted a new paper arXiv:0904.0784v1 [hep-th] on non-compact exceptional groups as quasiconformal groups over the split cubic Jordan algebras. The exceptional groups E_{6(6)}, E_{7(7)}, E_{8(8)} are known to arise from toroidal compactifications of D=11 supergravity down to d dimensions, where the global non-compact symmetry group of the maximally extended supergravity is given by E_{11-d(11-d)} (arXiv:hep-th/0409263v1). It is believed the discrete subgroups E_{6(6)}(Z), E_{7(7)}(Z), E_{8(8)}(Z) yield the symmetries of the non-perturbative spectra of toroidally compactified M-theory (arXiv:hep-th/9410167v2).

Algebraically, E_{6(6)}, E_{7(7)}, E_{8(8)} arise as transformation groups of the Freudenthal triple system over the split octonions. E_{6(6)} is the subgroup of the automorphism group of the Freudenthal triple system, E_{7(7)}, which preserves the cubic form of the split exceptional Jordan algebra. Geometrically, E_{6(6)} is the collineation group of the split Moufang plane OP^2_s, the space of projectors of the split exceptional Jordan algebra. E_{7(7)} and E_{8(8)} act as conformal and quasiconformal groups, respectively, over this space.

Gunaydin and Pavlyk show in their new paper that E_{6(6)}, E_{7(7)}, E_{8(8)} individually act as quasiconformal groups over the split cubic Jordan algebras J(3,C_s), J(3,H_s) and J(3,O_s). This implies they also act as quasiconformal groups over the split projective planes CP^2_s, HP^2_s and OP^2_s, respectively. Physically, this yields a new type of duality, a duality of U-dualities, so to speak. For example, M-theory compactified on an 8-torus with D=3 U-duality group E_{6(6)} is dual to M-theory compactified on a 6-torus with D=5 U-duality group E_{6(6)} as the symmetries of the non-perturbative spectra are equivalent.

Thursday, April 09, 2009

It is well known that inflation provides the most compelling solution to many long-standing problems of the big bang model (horizon, flatness, monopoles, etc.) The source of inflation is a scalar field (the inflaton field), which in light of modern theories, can be based on a Dirac-Borne-Infeld action rather than the conventional Klein-Gordon action, allowing the inflaton field to take the form of a tachyon field. A rolling tachyon field is usually associated with unstable D-brane configurations, where decay of such D-branes (via tachyon condensation) produces a pressureless gas with finite energy density resembling classical dust. In their April 6th paper (arXiv:0904.1032v1), del Campo et al. investigate such a tachyonic inflationary universe model in the context of intermediate inflation.

Friday, March 27, 2009

Yesterday, a striking new paper arXiv:0903.4630v1 emerged on the arxiv by the well-known Renata Kallosh. Kallosh builds on the unitary cut and pure spinor methods which have established 3-loop superfiniteness and suggested the onset of divergences starting from the 9-loop order or even all loop ﬁniteness, respectively. She specifically investigates N=8 supergravity on the light cone and makes her prediction for the actual computations using Feynman light-cone supergraphs. It is noted that light-cone superﬁeld amplitudes have a non-local structure in the transverse directions leading to the increase of the delay of divergences with the increasing number of legs in the loop-amplitudes. The delay of divergences leads to the all loop ﬁniteness prediction for N =8 supergravity.

Wednesday, February 18, 2009

Kalousios et al. posted a paper (arXiv:0902.3179) today investigating a new dyonic magnon solution on CP^3. Fans of twistor theory will recall that CP^3 is actually projective twistor space. However, here the context is N=6 superconformal Chern-Simons theory with SU(N)xSU(N) symmetry, which in the large N,k limit is dual to type IIA string theory on AdS_4 x CP^3. The present paper generalizes Maldacena's giant magnon construction where it is a particular open string configuration on an R x S^2 subset of AdS_5 x S^5. This is done by noticing the equations of motion for a string on R x CP^3 are classically integrable, and supplementing them with the Virasoro constraints. Various solutions are given, such as a pointlike string moving along the equator of an S^2 in CP^3 and another on a CP^1 which is a rotation of the ordinary Hofman-Maldacena giant magnon.

Looking back at twistor string theory, Witten wanted to consider the open string B-model on CP^3, but was hindered by the fact that CP^3 is not a Calabi-Yau manifold, hence giving an anomalous R-symmetry. For this reason he considered supermanifolds of the form CP^{3|N}, which is Calabi-Yau if and only if N=4.

In the present case, type IIA string theory has no such restrictions, so it might be that Kalousios et al. have laid the foundation for a new twistor string theory on ordinary CP^3, where the perturbative expansion of the superconformal N=6 theory could be related to a D-instanton expansion of the dual string theory on AdS_4 x CP^3.

We argue that the main feature behind novel properties of higher-dimensional black holes, compared to four-dimensional ones, is that their horizons can have two characteristic lengths of very different size. We develop a long-distance worldvolume effective theory that captures the black hole dynamics at scales much larger than the short scale. In this limit the black hole is regarded as a blackfold: a black brane (possibly boosted locally) whose worldvolume spans a curved submanifold of the spacetime. This approach reveals black objects with novel horizon geometries and topologies more complex than the black ring, but more generally it provides a new organizing framework for the dynamics of higher-dimensional black holes.

New Twistor String Theories RevisitedA gauged version of Berkovits twistor string theory featuring the particle content of N=8 supergravity was suggested by Abou-Zeid, Hull and Mason. The equations of motion for a particular multiplet in the modified theory are examined on the level of basic twistor fields and thereby shown to imply the vanishing of the negative helicity graviton on-shell. Additionally, the restrictions emerging from the equation of motion for the new gauge field \bar{B} reveal the chiral nature of interactions in theories constructed in this manner. Moreover, a particular amplitude in Berkovits open string theory is shown to be in agreement with the corresponding result in Einstein gravity.

In these lectures, I review the current status of cosmic strings and cosmic superstrings. I first discuss topological defects in the context of Grand Unified Theories, focusing in particular in cosmic strings arising as gauge theory solitons. I discuss the reconciliation between cosmic strings and cosmological inflation, I review cosmic string dynamics, cosmic string thermodynamics and cosmic string gravity, which leads to a number of interesting observational signatures. I then proceed with the notion of cosmic superstrings arising at the end of brane inflation, within the context of brane-world cosmological models inspired from string theory. I discuss the differences between cosmic superstrings and their solitonic analogues, I review our current understanding about the evolution of cosmic superstring networks, and I then briefly describe the variety of observational consequences, which may help us to get an insight into the stringy description of our Universe.

Tuesday, January 06, 2009

Happy 2009 to all U-duality readers! Today was the first day of class for the Winter term of Caltech's math sequence for theoretical physicists. This math sequence consists of three classes covering the modern mathematics a young string theorist should know. For the Fall term the main textbook was Nakahara's Geometry Topology and Physics, 2nd Edition.

This term, the main text is Morita's Geometry of Differential forms.

Today's lecture jumped right in to Čech cohomology, which can be thought of as a type of 'unifying' cohomology in the sense that it is equivalent to de Rham, simplicial and singular cohomology for well-behaved choices of topological space X. de Rham cohomology arises most frequently for physicists, but since it involves solving differential equations, it is not always the most pleasant for computations. Simplicial cohomology is probably the most computation-friendly cohomology, so Čech cohomology can be thought of as a kind of 'bridge' for translating physics problems from de Rham to a simpler (pun intended) simplicial cohomology setting. The proof of the equivalence of de Rham and Čech cohomology is most enlightening in this respect and involves forming a square of commutative (or anticommutative if you wish) mappings between vector spaces of bi-degree forms e.g. (p,q where total degree is p+q), with the vertical direction increasing via the de Rham coboundary operator, d, and horizontal direction via the Čech coboundary operator, delta. One can then define a new coboundary operator D=d+delta and show it satisfies D^2=0. I'll sketch the proof in more depth in another post, for those interested. I'll also refer the interested reader to another classic text on cohomology, namely Bott and Tu's, Differential Forms in Algebraic Topology.